Process For Downstream Recovery Of Nitroalkane Using Dividing Wall Column

ABSTRACT

Disclosed are a process and apparatus for synthesizing nitroalkanes by reaction of a hydrocarbon feedstock with aqueous nitric acid. Energy and capital costs may be reduced by using a dividing wall column.

FIELD

The invention relates to a process for synthesizing nitroalkanes. Morespecifically, the invention relates to a process for intensifiednitroalkane recovery in which a dividing wall column is used.

BACKGROUND

The nitration of hydrocarbons produces a variety of products, dependingon the reaction conditions and the feedstock compositions. For example,vapor phase nitration of propane typically results in a mixture of fournitro-paraffin products: nitromethane, 1-nitropropane, 2-nitropropane,and nitroethane in essentially fixed relative concentrations. Highpressure nitration of propane can selectively produce 2-nitropropaneover other lower molecular weight nitroalkanes like 1-nitropropane,nitromethane, and nitroethane. The high pressure nitration ofcyclohexane typically results in the formation of cyclohexanol,cyclohexanone, nitrocyclohexane, and oxidation products.

The byproducts from typical vapor phase and high pressure nitration ofpropane, can have similar boiling points, especially among the lowmolecular weight nitroalkanes (2-nitropropane, 1-nitropropane,nitromethane, and nitroethane), making separation difficult. Aconventional post-reaction distillation sequence uses tall columns andhigh reflux ratios, which are expensive and consume a lot of energy.Other conventional separation methods use an additional mass-separatingagent to recover the desired nitro-paraffin. A need exists, therefore,for more economical and energy efficient processes for recoveringdesired nitroalkane products.

BRIEF SUMMARY

In one aspect, a process is provided for synthesizing at least onenitroalkane. The process comprises: reacting in a reactor a hydrocarbonfeedstock with aqueous nitric acid, such that a product stream isproduced; separating the product stream into at least an oil phase andan aqueous phase; removing substantially all organic acids from the oilphase; thereafter, distilling the oil phase in a dividing wall column,to recover at least a top product, a middle product, and a bottomproduct; and recovering the at least one nitroalkane from the middleproduct.

In another aspect, a process for nitroalkane recovery is provided. Theprocess comprises: separating a product stream from a nitroparaffinnitration process into at least an oil phase and an aqueous phase;distilling the oil phase in a dividing wall column, to recover at leasta top product, a middle product, and a bottom product; recovering atleast a first nitroalkane from the middle product; and recovering atleast a second nitroalkane from the bottom product.

In yet another aspect, an apparatus for synthesizing at least onenitroalkane is provided. The apparatus comprises: a reactor for reactinga hydrocarbon feedstock with aqueous nitric acid to form a reactionproduct stream; a phase separation apparatus for separating the reactionproduct stream into at least an oil phase and an aqueous phase; and adividing wall column for distilling the oil phase into at least a topproduct, a middle product, and a bottom product, wherein the middleproduct comprises at least a portion of the at least one nitroalkane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an apparatus for synthesizing at leastone nitroalkane, in accordance with an illustrative embodiment.

FIG. 2 is a schematic diagram of an apparatus for synthesizing at leasta first nitroalkane and a second nitroalkane, in accordance with anillustrative embodiment.

FIG. 3 is a schematic diagram of an apparatus for synthesizing at leastone nitroalkane, in accordance with an illustrative embodiment.

FIG. 4 is a graph of the mole fraction of 2-nitropropane in a topproduct as a function of liquid split ratio and duty for a high pressurenitration.

FIG. 5 is a graph of the mole fraction of nitroethane in a middleproduct as a function of liquid split ratio and duty for a high pressurenitration.

FIG. 6 is a graph of the mole fraction of 1-nitropropane in a middleproduct as a function of liquid split ratio and duty for a high pressurenitration.

FIG. 7 is a graph of the mole fraction of 2-nitropropane in a bottomproduct as a function of liquid split ratio and duty for a high pressurenitration.

FIG. 8 is a graph of the impurity mole fraction as a function of liquidsplit ratio at an energy ratio of 881 BTU/lb for a high pressurenitration.

FIG. 9 is a graph of the mole fraction of 2-nitropropane in a topproduct as a function of liquid split ratio and duty for a vapor phasenitration.

FIG. 10 is a graph of the mole fraction of nitroethane in a middleproduct as a function of liquid split ratio and duty for a vapor phasenitration.

FIG. 11 is a graph of the mole fraction of 1-nitropropane in a middleproduct as a function of liquid split ratio and duty for a vapor phasenitration.

FIG. 12 is a graph of the mole fraction of 2-nitropropane in a bottomproduct as a function of liquid split ratio and duty for a vapor phasenitration.

FIG. 13 is a graph of the impurity mole fraction as a function of liquidsplit ratio at an energy ratio of 1855 BTU/lb for a vapor phasenitration.

FIG. 14 is a graph of the mole fraction of 2-nitropropane in a topproduct as a function of liquid split ratio and duty for a modified highpressure nitration.

FIG. 15 is a graph of the mole fraction of nitroethane in a middleproduct as a function of liquid split ratio and duty for a modified highpressure nitration.

FIG. 16 is a graph of the mole fraction of 1-nitropropane in a middleproduct as a function of liquid split ratio and duty for a modified highpressure nitration.

FIG. 17 is a graph of the mole fraction of 2-nitropropane in a bottomproduct as a function of liquid split ratio and duty for a modified highpressure nitration.

FIG. 18 is a graph of the impurity mole fraction as a function of liquidsplit ratio at an energy ratio of 923 BTU/lb for a modified highpressure nitration.

DETAILED DESCRIPTION

In one aspect, a process for synthesizing at least one nitroalkane isprovided. This process may beneficially use a dividing wall columninstead of two distillation columns to recover at least one nitroalkane,therefore reducing the capital and energy costs associated with thesynthesis of nitroalkanes.

FIG. 1 illustrates an apparatus 100 for synthesizing at least onenitroalkane. A hydrocarbon feedstock 101 and aqueous nitric acid 102 maybe introduced into a reactor 103. The hydrocarbon feedstock 101 and theaqueous nitric acid 102 may react at a reactor pressure and a reactiontemperature, such that a product stream 104 comprising nitratedcompounds and byproducts may be formed.

The hydrocarbon feedstock 101 and the aqueous nitric acid 102 may bemixed, or partially mixed, prior to entry into the reactor 103 or,alternatively; they may be added individually, with mixing to occurwithin the reactor 103. Further, hydrocarbon feedstock 101 and theaqueous nitric acid 102, whether added together or individually, may bepreheated prior to entry into the reactor 103.

In one example, the hydrocarbon feedstock 101 may consist essentially ofpropane and acetic acid. In other examples, the hydrocarbon feedstock101 may include, without limitation, one or more of the following:alkanes and cycloalkanes (including alkyl substituted cycloalkanes),such as propane, isobutane, n-butane, isopentane, n-pentane, n-hexane,n-heptane, n-octane, 2,3-dimethylbutane, cyclohexane, cyclopentane, andmethylcyclohexane; aryl alkanes such as ethylbenzene, toluene, xylenes,isopropyl benzene; 1-methylnaphthalene and 2-methylnaphthalene and4-methylbiphenyl; fused cycloalkanes; alkyl substituted fused arylcompounds; fused cyclolalkane-aryl compounds (including alkylsubstituted derivatives), such as tetralin, decalin, andmethylnaphthalene; and carboxylic acids, such as acetic acid, propanoicacid, butanoic acid, and hexanoic acid. The nitration of reactants thatalready have one or more nitro substituents is also contemplatedprovided that the reactant still has an available hydrogen.

The aqueous nitric acid may be delivered to the reactor 103 in the formof an aqueous solution that contains at least about 10 weight percent,preferably at least about 15 weight percent, more preferably at leastabout 20 weight percent, of the acid. Further, the solution may containless than about 50 weight percent, preferably less than about 40 weightpercent, and more preferably less than about 35 weight percent, of theacid. In further embodiments, the nitric acid solution may containbetween about 15 and about 40 weight percent of the acid. In otherembodiments, the nitric acid solution may contain between about 18 andabout 35 weight of the acid.

The mole ratio of they hydrocarbon feedstock 101 to the aqueous nitricacid 102 may be at least about 0.3:1, more preferably at least about0.5:1.

The reactor pressure may be at least about 500 psi (34 atm), preferablyat least about 1000 psi (68 atm), and more preferably at least about1200 psi (82 atm). Further, the pressure may be less than about 1600 psi(109 atm), preferably less than about 1500 psi (102 atm), and morepreferably less than about 1400 psi (95 atm). In other embodiments, thepressure may between about 1000 psi (68 atm) and 1400 psi (95 atm).Various methods known in the art may be used for maintaining thepressure within the desired range including, for example, through theuse of a back-pressure regulator.

The reaction temperature within the reactor may be controlled (forexample with heat exchange fluid or using heat generated from thereaction) to greater than about 140 degrees Celsius and less than about325 degrees Celsius. In other embodiments, the temperature may begreater than about 215 degrees Celsius and less than about 325 degreesCelsius. In some embodiments, the temperature may be greater than about180 degrees, greater than about 200 degrees, greater than about 230degrees, or greater than about 240 degrees. In further embodiments, thetemperature may be less than about 290 degrees, less about 280 degrees,less than about 270 degrees, or less than about 250 degrees. In otherembodiments, the temperature may be between about 200 and 250 degreesCelsius. In yet further embodiments, the temperature may be betweenabout 215 and 280 degrees Celsius, or between about 220 and 270 degreesCelsius.

Residence time of the reactants in the reactor 103 may be preferably atleast about 30 seconds, more preferably at least about 90 seconds.Residence time may be controlled in various ways including, for example,by the length and/or width of the reactor or through the use of packingmaterial. Residence time may be determined by dividing the volume of thereactor by the inlet flow rates.

The reactor 103 may be a downflow configured reactor. That is, thereactor, which is preferably of an elongated and linear shape, such as atube shape, may be positioned so that reactants are added through anentry port at or near the top of the reactor and then flow down thereactor for a residence time that is sufficient to allow reaction tooccur and formation of the desired product. The product mixture may becollected through an exit port at or near the bottom of the reactor.

The operation of the reactor in a downflow configuration providescertain advantages over prior art systems, which generally utilize ahorizontal, upflow, coiled or a batch autoclave type apparatus. Inparticular, the downflow configuration of the invention providesnitrated compounds that contain relatively low levels of oxidationbyproducts as compared to such prior art systems.

Without wishing to be bound by any particular theory, it is believedthat the advantages of the downflow reactor result primarily from itsability to minimize the amount and residence time of the liquid phasewithin the reactor. The liquid phase in general contains a low moleratio of hydrocarbons to nitric acid. This low mole ratio favorsoxidation chemistry at the expense of nitration and oxidation thereforeprimarily occurs in the liquid phase. In a downflow reactor (alsoreferred to as a trickle bed reactor) the gas is the continuous phaseand the liquid trickles down the reactor walls or packing. Therefore,the amount of liquid phase(s) in a downflow configured reactor ismaintained at a low level and consequently oxidation chemistry isminimized.

In contrast, in an upflow reactor, also referred to as a bubble column,the liquid is the continuous phase (and bubbles rise quickly through thecontinuous liquid phase). Thus, an upflow reactor maximizes the liquidholdup. Because, as noted above, oxidation primarily occurs in theliquid phase, the upflow reactor maximizes the formation of oxidationbyproducts. Similarly, coil and horizontal reactor configurations alsoincrease liquid residence time and therefore oxidation chemistry ascompared to a downflow reactor. A further disadvantage of coiledreactors is that they are not well-suited for industrial scaleproduction because of the difficulty of fabricating large scale reactorsin this shape.

The reactor 103 may also be packed with a packing material to improvereactant mixing and heat transfer and/or to vary the reactor volume.Packing of the reactor may be preferred, for example, in a propanenitration system where it is desired to increase the concentration of2,2-dinitropropane in the product stream. Suitable packing materialsinclude, for example, glass beads, random packing, or structuredpacking, such as those typically employed in distillation devices. Otherpacking materials are known in the art and may be used.

The product stream 104 may then enter an absorber 105 for absorbingwater-soluble and oil soluble components from the product stream 104 toform a hydrocarbon gas stream 106 and a gas-recovered mixture 107. Thehydrocarbon gas stream 106 may contain unreacted hydrocarbons, carbonmonoxide, carbon dioxide, nitric oxide, nitrous oxide, and nitrogen. Theunreacted hydrocarbons in the hydrocarbon gas stream 106 may be recycledto the reactor 103. The remaining gases in the hydrocarbon gas stream106 may be processed to recover nitric oxide as nitric acid. The furtherremaining gases in the hydrocarbon gas stream 106 may be sent through aburner and emitted. The gas-recovered mixture 107 may then enter astripper 108 where an aqueous phase 109 may be stripped from thegas-recovered mixture 107 to form a second gas-recovered mixture 110.Next, the second gas-recovered mixture 110 may enter separator 111,where the second gas-recovered mixture 110 is separated into an oilphase 112 and a second aqueous phase 113. The second aqueous phase 113may be recycled back to the phase separation apparatus 108. The aqueousphase 109 from the phase separation apparatus 108 may be disposed of orfurther recycled back to the reactor 103. The oil phase 112 may thenenter a neutralization/water-wash apparatus 114 where substantially allthe organic acids may be removed from the oil phase 112, resulting in atleast a neutralized oil phase 115 and a waste stream 116. Theneutralized oil phase 115 then may enter a dividing wall column 117,which may distill the neutralized oil phase 115.

The dividing wall column 117 may be operated at a vapor split ratio ofbetween about 0.3:0.7 and 0.7:0.3, preferably between about 0.4:0.6 and0.6:0.4, and more preferably about 0.5:0.5. The dividing wall column 117may be operated at a liquid split ratio of between about 0.2:0.8 and0.8:02, preferably between about 0.3:0.7 and 0.7:0.3, and morepreferably between about 0.3:0.7 and 0.5:0.5. In other embodiments, theliquid split ratio may be between about 0.35:0.65 and 0.4:0.6,preferably about 0.37:0.63. In yet other embodiments, the liquid splitratio may be preferably about 0.35:0.65. In further embodiments, theliquid split ratio may be between about 0.45:0.65 and 0.5:0.5,preferably about 0.46:0.54.

The dividing wall column 117 may include a condenser 118 and a reboiler119. The condenser 118 may be operated at a temperature between 20 and80 degrees Celsius. The reboiler 119 may be operated at a temperaturebetween 75 and 85 degrees Celsius.

The dividing wall column 117 may recover at least a top product 120, amiddle product 121, and a bottom product 122. At least one nitroalkanemay be recovered from the middle product 121. Examples of nitroalkanesthat may be recovered include, among others, 2-nitropropane andnitrocyclohexane.

FIG. 2 illustrates an apparatus 200 for synthesizing one or morenitroalkanes. A hydrocarbon feedstock 201 and aqueous nitric acid 202may be introduced into a reactor 203. Reactor 203 may be similar toreactor 103 in FIG. 1. The hydrocarbon feedstock 201 and the aqueousnitric acid 202 may react at a reactor pressure and a reactiontemperature, such that a product stream 204 comprising nitratedcompounds and byproducts may be formed.

The hydrocarbon feedstock 201 and the aqueous nitric acid 202 may bemixed, or partially mixed, prior to entry into the reactor 203 or,alternatively; they may be added individually, with mixing to occurwithin the reactor 203. Further, hydrocarbon feedstock 201 and theaqueous nitric acid 202, whether added together or individually, may bepreheated prior to entry into the reactor 203.

The hydrocarbon feedstock 201 may be similar to the hydrocarbonfeedstock 101 in FIG. 1. The aqueous nitric acid 202 may have a similarcomposition as the aqueous nitric acid 102 in FIG. 1. The mole ratio ofthey hydrocarbon feedstock 201 to the aqueous nitric acid 202 may be atleast about 0.3:1, more preferably at least about 0.5:1. The reactorpressure, reaction temperature, and residence time may be similar tothat in FIG. 1.

The product stream 204 may then enter an absorber 205 for absorbingwater-soluble and oil soluble components from the product stream 204 toform a hydrocarbon gas stream 206 and a gas-recovered mixture 207. Thehydrocarbon gas stream 206 may contain unreacted hydrocarbons, carbonmonoxide, carbon dioxide, nitric oxide, nitrous oxide, and nitrogen. Theunreacted hydrocarbons in the hydrocarbon gas stream 206 may be recycledto the reactor 203. The remaining gases in the hydrocarbon gas stream206 may be processed to recover nitric oxide as nitric acid. The furtherremaining gases in the hydrocarbon gas stream 206 may be sent through aburner and emitted. The gas-recovered mixture 207 may then enter astripper 208 where an aqueous phase 209 may be stripped from thegas-recovered mixture 207 to form a second gas-recovered mixture 210.Next, the second gas-recovered mixture 210 may enter separator 211,where the second gas-recovered mixture 210 is separated into an oilphase 212 and a second aqueous phase 213. The second aqueous phase 213may be recycled back to the phase separation apparatus 208. The aqueousphase 209 from the phase separation apparatus 208 may be disposed of orfurther recycled back to the reactor 203. The oil phase 212 may thenenter a neutralization/water-wash apparatus 214 where substantially allthe organic acids may be removed from the oil phase 212, resulting in atleast a neutralized oil phase 215 and a waste stream 216. Theneutralized oil phase 215 then may enter a dividing wall column 217,which may distill the neutralized oil phase 215.

The neutralized oil phase 215 may be similar to the neutralized oilphase 115 in FIG. 1. The dividing wall column 217 may be similar to thedividing wall column 117 in FIG. 1, with a condenser 218 and a reboiler219.

The dividing wall column 217 may recover at least a top product 220, amiddle product 221, and a bottom product 222. At least one nitroalkanemay be recovered from the middle product 221. Examples of nitroalkanesthat may be recovered include, among others, 2-nitropropane andnitrocyclohexane.

The bottom product 222 may include additional nitroalkanes. For example,the bottom product 222 may include 1-nitropropane, nitromethane, andnitroethane. The bottom product may enter a distillation column 223 torecover at least one additional nitroalkane. The distillation column 223may distill the bottom product 222 such that at least a second topproduct 224 and a second bottom product 225 are produced. The second topproduct 224 may comprise an additional nitroalkane, for example,1-nitropropane.

FIG. 3 illustrates an apparatus 300 for synthesizing at least onenitroalkane. A hydrocarbon feedstock 301 and aqueous nitric acid 302 maybe introduced into a reactor 303. Reactor 303 may be similar to reactor103 in FIG. 1. The hydrocarbon feedstock 301 and the aqueous nitric acid302 may react at a reactor pressure and a reaction temperature, suchthat a product stream 304 comprising nitrated compounds and byproductsmay be formed.

The hydrocarbon feedstock 301 and the aqueous nitric acid 302 may bemixed, or partially mixed, prior to entry into the reactor 303 or,alternatively; they may be added individually, with mixing to occurwithin the reactor 303. Further, hydrocarbon feedstock 301 and theaqueous nitric acid 302, whether added together or individually, may bepreheated prior to entry into the reactor 303.

The hydrocarbon feedstock 301 may be similar to the hydrocarbonfeedstock 101 in FIG. 1. The aqueous nitric acid 302 may have a similarcomposition as the aqueous nitric acid 102 in FIG. 1. The mole ratio ofthey hydrocarbon feedstock 301 to the aqueous nitric acid 302 may be atleast about 0.3:1, more preferably at least about 0.5:1. The reactorpressure, reaction temperature, and residence time may be similar tothat in FIG. 1.

The product stream 304 may then enter an absorber 305 for absorbingwater-soluble and oil soluble components from the product stream 304 toform a hydrocarbon gas stream 306 and a gas-recovered mixture 307. Thehydrocarbon gas stream 306 may contain unreacted hydrocarbons, carbonmonoxide, carbon dioxide, nitric oxide, nitrous oxide, and nitrogen. Theunreacted hydrocarbons in the hydrocarbon gas stream 306 may be recycledto the reactor 303. The remaining gases in the hydrocarbon gas stream306 may be processed to recover nitric oxide as nitric acid. The furtherremaining gases in the hydrocarbon gas stream 306 may be sent through aburner and emitted. The gas-recovered mixture 307 may then enter astripper 308 where an aqueous phase 309 may be stripped from thegas-recovered mixture 307 to form a second gas-recovered mixture 310.Next, the second gas-recovered mixture 310 may enter separator 311,where the second gas-recovered mixture 310 is separated into an oilphase 312 and a second aqueous phase 313. The second aqueous phase 313may be recycled back to the phase separation apparatus 308. The aqueousphase 309 from the phase separation apparatus 308 may be disposed of orfurther recycled back to the reactor 303. For example, the aqueous phase309 may contain organic acids, such as acetic acid, which may bereturned to the reactor 303. The oil phase 312 may then enter aneutralization/water-wash apparatus 314 where substantially all theorganic acids may be removed from the oil phase 312, resulting in atleast a neutralized oil phase 315 and a waste stream 316. Theneutralized oil phase 315 then may enter a dividing wall column 317,which may distill the neutralized oil phase 315.

The neutralized oil phase 315 may be similar to the neutralized oilphase 115 in FIG. 1. The dividing wall column 317 may be similar to thedividing wall column 117 in FIG. 1, with a condenser 318 and a reboiler319.

The dividing wall column 317 may recover at least a top product 320, amiddle product 321, and a bottom product 322. At least one nitroalkanemay be recovered from the middle product 321. Examples of nitroalkanesthat may be recovered include, among others, 2-nitropropane. In anillustrative embodiment, at least one nitroalkane is recovered from thetop product 320. Examples of nitroalkanes that may be recovered include,among others, nitromethane.

In other illustrative embodiments, the neutralized oil phase 315 mayenter a recovery apparatus for recovering a hydrocarbon prior toentering the dividing wall column 317. The recovery apparatus mayrecover, for example, cyclohexane, from the neutralized oil phase 315.

EXAMPLES

Various examples are demonstrated using a computer simulation.

Example 1 High Pressure Nitration of Propane

Propane is reacted with 30 weight percent aqueous nitric acid at areactor pressure of about 1200 psi (77.4 atm), an average reactiontemperature of about 250 degrees Celsius (a range of 220 to 290 degreesCelsius), a residence time of about 120 seconds, and a propane to nitricacid mole ratio of about 1.5:1 in a high pressure nitration process toproduce a product stream. This product stream is then sent to a dividingwall column (DWC). The major components of the product stream feed tothe DWC are summarized in Table 1 below. The scheme is designed for a2-nitropropane production rate of 5420 lb/h. and consists of arelatively small fraction of components lighter than 2-nitropropane,followed by a large mole fraction of the desired product 2-nitropropane,and then a relatively small amount of components heavier than2-nitropropane.

TABLE 1 Feed to the DWC in a high pressure nitration processTemperature, ° C. 21 Pressure, atm 1 Mass flow, lb/h 6071.8 Molefraction Water 0.031554 Nitrous oxide 0.000254 Propane 0.020788 Acetone0.027185 Butane 0.003291 Nitromethane 0.014159 Nitroethane 0.0077742-nitropropane 0.799292 1-nitropropane 0.072607 1-nitrobutane 5.56E−052-nitrobutane 0.000223 2,2-dinitropropane 0.022654 Kerosene 0.000158

The DWC is operated to separate the product stream into a top product, amiddle product, and a bottom product. The top product from the DWC isessentially the volatiles (nitromethane, nitroethane, and acetone) alongwith water, the middle product is essentially pure 2-nitropropane, andthe bottom product is essentially 1-nitropropane and heavies(nitrobutane, 2,2-dinitropropane, and kerosene). The DWC can be followedby another column to recover 1-nitropropane from the bottom product.

The desired purity of 2-nitropropane stream is 99.6% with a >99.5%recovery. The DWC design to achieve these specifications is shown belowin Table 2. A total of 84 stages are required, with the feed stage atthe 51^(st) stage from the condenser. The 2-nitropropane productdraw-off is taken off from the 43^(rd) stage from the condenser.

The impurities include: 2-nitropropane in the top product, nitroethaneand 1-nitropropane in the middle product, and 2-nitropropane in thebottom product. The impurity levels decrease as the reboiler duty isincreased, as shown in FIGS. 4-7. FIGS. 4-7 illustrate the variation ofthe above impurities with respect to the percentage of liquid split thatis sent to the pre-fractionator and the ratio of heat duty to feed, on aunit ratio basis (BTU/lb). The vapor split ratio is fixed to 50:50whereas the optimum liquid split ratio with respect to reboiler duty is37:63, as shown in FIG. 8. FIG. 8 illustrates a magnified section forthe variation of impurity with liquid split ratio (0.35 to 0.40) at theenergy unit ratio of 881 BTU/lb (a reboiler duty of 5.35 MMBTU/h). Theminimum amount of 1-nitropropane impurity in the middle product and the2-nitropropane impurity in the bottom product is at 37 percent of liquidfeed to the pre-fractionator, therefore the optimum liquid split ratiofor this example is 0.37:0.63.

Table 2 compares the simulation results using the product streamsummarized in Table 1 above for a DWC set-up as compared to using theproduct stream in Table 1 for conventional scheme, using twodistillation columns in the finishing train. The conventional sequenceconsumes 7.32 MM BTU/hour for the separation requirements, using a totalof 106 stages within the two distillation columns. The energy savings ofthe DWC are thus approximately 29% over the direct sequence. The singleDWC that replaces both columns in the direct sequence is larger indiameter and area than either of the two columns it replaces, but onlyslightly so. A single DWC of 2.4 meters (8.15 feet) diameter replacesboth the direct sequence columns.

TABLE 2 Comparison of DWC in high pressure nitration with directsequence Divided Wall Direct Scheme Column Column 1 Column 2 Eqm. No. ofstages 84 50 56 Pressure, atm 0.165 0.165 0.165 Top temperature, ° C.28.87 29.03 67 Bottom temperature, ° C. 81.41 68.22 81.6 Feed stage(from top) 51 33 32 Product stage 43 — — Liquid split ratio 0.37:0.63Vapor split ratio 0.5:0.5 Reboiler duty, 5.35 3.77 3.71 MMBTU/hCondenser duty, −5.04 −3.47 −3.7 MMBTU/h Column diameter, m 1.75 1.471.45 Volatiles, lbmol/h 7.53 7.53 Water (mole fraction) 0.297702640.297668 Nitrous oxide 0.00240066 0.0024 Propane 0.19619249 0.196169Acetone 0.25649699 0.256467 Butane 0.03104843 0.031045 Nitromethane0.1335381 0.133563 Nitroethane 0.05980164 0.060056 2-nitropropane0.0227673 0.02258 2-nitropropane stream, 56.794 56.794 lbmol/hNitroethane (mole 0.00179863 0.001761 fraction) 2-nitropropane0.99635095 0.996478 1-nitropropane 0.00184499 0.001761 1-nitropropanestream, 7.19 7.19 lbmol/h 2-nitropropane (mole 0.0014739 0.000683fraction) 1-nitropropane 0.70303699 0.703813 1-nitrobutane 0.000549580.00055 2-nitrobutane 0.0022033 0.002203 2,2-dinitropropane 0.291171520.291186 Kerosene 0.00156467 0.001565

Example 2 Vapor Phase Nitration of Propane

Vapor phase nitration has a lower selectivity towards 2-nitropropane.Propane is reacted with 70 weight percent aqueous nitric acid at areactor pressure of about 185 psi (9.7 atm), an average reactiontemperature of about 370 degrees Celsius, a residence time of about 2.3seconds, and a propane to nitric acid mole ratio of about 4:1 in a vaporphase nitration process to produce an product stream. Table 3 summarizesa typical composition of the product stream coming out of the water-washneutralization section from a vapor phase nitration process comparedwith the stream composition of a similar stream from a high pressurenitration process.

TABLE 3 Composition of key components in input to the downstreampurification section of vapor phase nitration and high pressurenitration process Weight % Components Vapor High Pressure Nitromethane20.4 1.01 Nitroethane 5.0 0.6 2-nitropropane 56.5 83.3 1-nitropropane15.7 7.6

This product stream is then sent to a dividing wall column (DWC). Themajor components of the product stream feed to the DWC are summarized inTable 4 below.

TABLE 4 Feed to the DWC in a vapor phase nitration process Temperature,° C. 21 Pressure, atm 1 Mass flow, lb/h 2343.87 Mole fraction Water0.0853 Nitromethane 0.2513 Nitroethane 0.0502 2-nitropropane 0.47791-nitropropane 0.1323 2-nitrobutane 0.0029

In vapor phase nitration, more of the lower nitroalkanes (nitromethane,nitroethane, and 1-nitropropane) are formed as compared to high pressurenitration and therefore the weight fraction of 2-nitropropane issignificantly less. The energy and capital benefits as a result of a DWCare directly proportional to the weight fraction of the middle componentin the feed stream, which in this case is 2-nitropropane.

Similar to Example 1, the impurities include: 2-nitropropane in the topproduct, nitroethane and 1-nitropropane in the middle product, and2-nitropropane in the bottom product. The impurity levels decrease asthe reboiler duty is increased, as shown in FIGS. 9-12. FIGS. 9-12illustrate the variation of the above impurities with respect to thepercentage of liquid split that is sent to the pre-fractionator and theratio of heat duty to feed, on a unit ratio basis (BTU/lb). The vaporsplit ratio is fixed to 50:50 whereas the optimum liquid split ratiowith respect to reboiler duty is 46:54, as shown in FIG. 13. FIG. 13illustrates a magnified section for the variation of impurity withliquid split ration (0.45 to 0.5) at the energy unit ratio of 1835BTU/lb (a reboiler duty of 4.35 MMBTU/h). The minimum amount of2-nitropropane impurity in the top product and nitroethane impurity inthe middle product is at a 47 percent liquid feed to thepre-fractionator, but the amount of 1-nitropropane impurity in themiddle product and the 2-nitropropane impurity in the bottom productrise steeply beyond 46 percent, therefore the optimum liquid split ratiofor this example is 0.46:0.64.

A comparison of a proposed DWC separation using the product streamsummarized in Table 4 above with a conventional direct scheme using theproduct stream in Table 4 is shown in Table 5 below.

TABLE 5 Comparison of DWC scheme with Direct Scheme for vapor phasenitration Divided Wall Direct Sequence Column Column 1 Column 2 Eqm. No.of stages 72 46 41 Pressure, atm 0.165 0.165 0.165 Top temperature, ° C.41.7 47.5 66.9 Bottom temperature,° C. 77.6 68.9 77 Feed stage (fromtop) 45 31 24 Product stage 41 — — Liquid split ratio 0.46:0.54 — —Vapor split ratio 0.5:0.5 — — Reboiler duty, 4.35 4.71 1.1 MMBTU/hCondenser duty, −4.1 −4.45 −1.1 MMBTU/h Column diameter, m 2.1 1.64 0.78Volatiles, lbmol/h 12.0638 12.0643 — Water (mole fraction) 0.2199 0.2199— Nitromethane 0.6483 0.6483 — Nitroethane 0.1234 0.1235 —2-nitropropane 0.0083 0.0083 — 2-nitropropane stream, 14.7217 — 14.7217lbmol/h Nitromethane (mole 1.3E−06 — 8.5E−08 fraction) Nitroethane0.0049 — 0.0049 2-nitropropane 0.9899 — 0.9903 1-nitropropane 0.0051 —0.0048 1-nitropropane stream, 4.3334 — 4.3328 lbmol/h Nitroethane (mole3.4E−06 4.8E−06 fraction) 2-nitropropane 0.0459 — 0.0447 1-nitropropane0.9330 — 0.9342 2-nitrobutane 0.0211 — 0.0211

According to the direct scheme, a 99.05% pure 2-nitropropane productspecification requires two columns with 87 total equilibrium stages,whereas a single dividing wall column with 72 equilibrium stagesachieves the same specifications utilizing 25% less energy. The feed tothe DWC enters at the 45^(th) stage from the condenser and the2-nitropropane product draw-off is taken off from the 41^(st) stage fromthe condenser.

Example 3 Modified High Pressure Nitration

In a modified version of the high pressure process, the main oxidationbyproducts (carboxylic acids) are recycled to produce valuablenitroparaffin products rather than discarded to wastewater treatment.Propane is reacted with 30 weight percent nitric acid at a reactorpressure of about 1200 psi (77.4 atm), an average reaction temperatureof about 235 degrees Celsius (a range of 180 to 290 degrees Celsius), aresidence time of about 120 seconds, and a propane to nitric acid moleratio of about 0.5:1. The main byproduct of propane nitration is aceticacid in high pressure nitration (acetic acid may also be added atstartup in order to more quickly obtain a steady-state process). Thisbyproduct, when concentrated and recycled to the reactor yieldssignificant amounts of nitromethane at typical high pressure nitrationprocess conditions. Nitromethane selectivity is about 55%, which isabout 30% higher than the maximum achievable using the commercialtechnology. The composition of a typical product stream coming out ofthe water-wash neutralization section of the modified high pressureprocess is summarized in Table 6 below and is compared with the highpressure and vapor phase (commercial process) streams.

TABLE 6 Composition of key components in input to the downstreampurification section of vapor phase nitration, high pressure nitrationand modified high pressure nitration process weight % High Modified HighComponents Pressure Vapor Pressure Nitromethane 1.01 20.4 55 Nitroethane0.6 5.0 1 2-nitropropane 83.3 56.5 40 1-nitropropane 7.6 15.7 4

The composition of the middle boiling component 2-nitropropaneprogressively decreases from high pressure to modified high pressureprocess; therefore the benefits due to using a DWC would also follow thesame trend. Nevertheless, the capital and energy savings still exist, asis shown in the analysis below. The product stream is then sent to adividing wall column (DWC). The major components of the product streamfeed to the DWC are summarized in Table 7 below.

TABLE 7 Feed to the DWC in a modified high pressure process Temperature,° C. 21 Pressure, atm 1 Mass flow, lb/h 5200 Mole fraction Nitromethane0.6398 Nitroethane 0.0095 2-Nitropropane 0.3188 1-Nitropropane 0.0319

Similar to Examples 1 and 2, the impurities include: 2-nitropropane inthe top product, nitroethane and 1-nitropropane in the middle product,and 2-nitropropane in the bottom product. The impurity levels decreaseas the reboiler duty is increased, as shown in FIGS. 14-17. FIGS. 14-17illustrate the variation of the above impurities with respect to thepercentage of liquid split that is sent to the pre-fractionator and theratio of heat duty to feed, on a unit ratio basis (BTU/lb). The vaporsplit ratio is fixed to 50:50 whereas the optimum liquid split ratiowith respect to reboiler duty is 46:54, as shown in FIG. 18. FIG. 18illustrates a magnified section for the variation of impurity withliquid split ration (0.3 to 0.4) at the energy unit ratio of 923 BTU/lb(a reboiler duty of 4.8 MMBTU/h). The minimum amount of 2-nitropropaneimpurity in the top product and nitroethane impurity in the middleproduct is at 36 percent liquid feed to the pre-fractionator, but theamount of 1-nitropropane impurity in the middle product and the2-nitropropane impurity in the bottom product rise steeply beyond 35percent, therefore the optimum liquid split ratio for this example is0.35:0.65

A comparison of the proposed DWC set-up using the product streamsummarized in Table 7 above with the conventional scheme using theproduct stream in Table 7 is shown in Table 8 below.

TABLE 8 DWC comparison with direct scheme for modified high pressureprocess Divided Wall Direct Scheme Column Column 1 Column 2 Eqm. No. ofstages 76 55 35 Pressure, atm 0.165 0.165 0.165 Top temperature, ° C.51.5 51.5 67 Bottom temperature, ° C. 76.14 67.7 76.11 Feed stage (fromtop) 46 42 20 Product stage 53 — — Liquid split ratio 0.35:0.65 — —Vapor split ratio 0.5:0.5 — — Reboiler duty, 4.8 4.7 1.5 MMBTU/hCondenser duty, −3.9 −3.8 −1.5 MMBTU/h Column diameter, m 2.3 1.65 0.92Volatiles, lbmol/h 47.5382 47.54685 — Nitromethane (mole 0.9856 0.9854 —fraction) Nitroethane 0.0123 0.0125 — 2-nitropropane 0.0021 0.0021 —2-nitropropane stream, 23.2226 — 23.2226 lbmol/h Nitromethane (mole2.8E−06 — 4.8E−06 fraction) Nitroethane 0.0047 — 0.0043 2-nitropropane0.9918 — 0.9910 1-nitropropane 0.0036 — 0.0047 1-nitropropane stream,2.4668 — 2.4582 lbmol/h Nitroethane (mole 5.3E−06 8.4E−06 fraction)2-nitropropane 0.0871 — 0.0946

According to the direct scheme, a 99.18% pure 2-nitropropane productspecification would require two columns consisting of a total of 90equilibrium stages, whereas a single dividing wall column with 76equilibrium stages achieves the same specifications utilizing about 22%less energy. The feed to the DWC enters at the 46^(th) stage from thecondenser and the 2-nitropropane product draw-off is taken off from the53^(rd) stage from the condenser. A reboiler duty of 4.8 MMBTU/h isrequired to process a feed stream of 5200 lb/h.

While the invention has been described above according to its preferredembodiments, it can be modified within the spirit and scope of thisdisclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using the generalprinciples disclosed herein. Further, the application is intended tocover such departures from the present disclosure as come within theknown or customary practice in the art to which this invention pertainsand which fall within the limits of the following claims.

What is claimed is:
 1. An apparatus for synthesizing at least onenitroalkane, the apparatus comprising: a reactor for reacting ahydrocarbon feedstock with aqueous nitric acid to form a reactionproduct stream; a phase separation apparatus for separating the reactionproduct stream into at least an oil phase and an aqueous phase; and adividing wall column for distilling the oil phase into at least a topproduct, a middle product, and a bottom product, wherein the middleproduct comprises at least a portion of the at least one nitroalkane. 2.A apparatus according to claim 1, the apparatus further comprising adistillation column for recovering at least a second nitroalkane fromthe bottom product.
 3. An apparatus according to claim 1, wherein the atleast one nitroalkane is 2-nitropropane.
 4. An apparatus according toclaim 2, wherein the second nitroalkane is 1-nitropropane.